Historically, most electronic products have been assembled using a solder material and a soldering process. This has always had disadvantages, and a number of new trends are revealing or exacerbating other disadvantages.
One set of disadvantages relates to solder materials themselves. Since the earliest days of the electronics industry tin/lead type solders (e.g., Sn63/Pb37) have been widely used. Unfortunately, both tin and especially lead have serious chemical disadvantages. For these two metals, mining the ores, refining it, working with the refined metals during manufacturing, being exposed to the substances in manufactured products, and disposing of the products at the ends of their life cycles are all potentially damaging to human health and to the environment.
Recently human health and environmental concerns about tin/lead type solders have resulted in the Directive on the Restriction of the Use of Certain Hazardous Substances in Electrical and Electronic Equipment (commonly referred to as the Restriction of Hazardous Substances Directive or RoHS) in the European Union. This directive restricts the use of six hazardous materials, including lead, in the manufacture of various types of electronic and electrical equipment. This directive is also closely linked with the Waste Electrical and Electronic Equipment Directive (WEEE) 2002/96/EC, which sets collection, recycling, and recovery targets for electrical goods. Together these directives are part of a growing world-wide legislative initiative to solve the problem of electronic device waste.
To some extent the electronics industry has always been searching for a practical substitute for tin/lead type solders, and legislative initiatives like those just noted are now motivating a number of changes. Today a common substitute for tin/lead type solders are SAC type solder varieties, which are alloys containing tin (Sn), silver (Ag), and copper (Cu). But this is merely a compromise. Mining, refining, working during manufacturing, exposure from manufactured products, and disposal are still all issues for tin, silver, and copper. It therefore suffices here to say that the undue use of some materials, like solder, is generally undesirable in electronic assemblies. Another set of disadvantages in the solder-based assembly of electronic products is the heat-based processes that are inherently required. The use of heat on and around many electronic components has always been undesirable. As a general principle, the heating of electronic components increases their failure rate and beyond a certain point outright damages such components. Tin/lead solders melt at relatively low temperatures, and their use has generally been tolerable for many components. This is not as frequently the same for SAC type solders, however, which melt at much higher temperatures. When SAC type solders are used, the likelihood of component damage is much higher, resulting in assemblies that fail during post-manufacturing testing as well as in-the-field failures. Additionally, generating and managing the heat during manufacturing have energy, safety, and financial costs. In addition, the undue use of lead-based manufacturing processes, like soldering, is also generally undesirable in electronic assemblies.
Increasingly yet another set of disadvantages in the solder-based assembly of electronic products is related to the “adding” of materials. When a material, like solder, is added between two components to hold them together, the additional material inherently has to occupy some space. The use of liquid-state materials, like solder in its liquid stage, in manufacturing often requires additional space around leads, terminals, and pads because both product and process designs need to account for the ability of liquid to flow easily and thus to potentially short to other leads, terminals, pads, etc. Liquid surface tension effects are also usually a major consideration in such designs, as liquid solders have high surface tensions. These all thus are factors as designers increasingly strive to miniaturize electronic assemblies. So the undue use of additional material, including solder, in manufactured assemblies and manufacturing processes is generally undesirable.
The density of components on circuit boards may be increased by building up in a vertical direction. Components may be stacked one on top of another. This type of assembly reduces size and cost. It is also amenable to manufacture by means of reverse-interconnection process (RIP) as disclosed in related U.S. patent application Ser. No. 12/119,287, ELECTRONIC ASSEMBLIES WITHOUT SOLDER AND METHODS FOR THEIR MANUFACTURE; U.S. patent application Ser. No. 12/163,870, ELECTRONIC ASSEMBLIES WITHOUT SOLDER AND METHODS FOR THEIR MANUFACTURE; PCT Patent Application No. PCT/US2008/065131, ELECTRONIC ASSEMBLIES WITHOUT SOLDER AND METHODS FOR THEIR MANUFACTURE; U.S. patent application Ser. No. 12/170,426, ELECTRONIC ASSEMBLIES WITHOUT SOLDER AND METHODS FOR THEIR MANUFACTURE; and U.S. patent application Ser. No. 12/182,043, ASSEMBLY OF ENCAPSULATED ELECTRONIC COMPONENTS TO A PRINTED CIRCUIT BOARD; hereby incorporated by reference in their entirety and assigned to the assignee of the present invention.